It is often necessary or desirable for a person to exercise a particular muscle or group of muscles. For example, when a muscle is damaged, such as through injury or surgery, it is important to exercise the muscle to prevent atrophy and to strengthen the muscle for normal use. Further, people exercise healthy muscles to increase strength and to maintain an active and healthy lifestyle, as well as to improve their appearance. Various routines have been developed to exercise different muscle groups by forcing the muscles to contract and extend under a load, such as by moving a free weight against the force of gravity or by moving a handle whose movement is resisted by an exercise machine.
One such exercise is known as a shoulder press. An exerciser sits upright on a seat and grasps a barbell at shoulder level. The exerciser pushes the barbell upward, extending his arms, and then lowers the barbell down. This exercise can be dangerous as the exerciser may drop the barbell. Further, the exerciser should have a partner to spot him in case he fails to lift the weight. When using free weights, the resistance provided by gravity is constant while the strength of the muscles varies over the range of motion. Consequently, the muscles are not fully loaded at each point over the range of motion.
During a shoulder press, the hands seek to follow a curved path inward as the weight is moved upward. This path cannot be followed when using a barbell because the hands are maintained at a fixed distance. Further, the user's head must be tilted back or forward during the lower portion of the stroke to clear the barbell. These deficiencies can be overcome by performing the exercise with dumbbells.
To overcome these difficulties, machines have been developed that simulate the exercise movements of a shoulder press. In one apparatus, a lever is hinged to a frame by a pivot behind the user. During the exercise stroke, the user's hands move from the shoulder longitudinally upward in parallel arcs, in planes perpendicular to the back of the user. A weight resistance, such as a weight plate or a weight stack, is operably engaged to the lever to resist the movement of the user's hands. The user's hands cannot move laterally during the exercise stroke, and the user cannot choose the path taken by his hands.
Certain facilities purchasing exercise equipment prefer machines with free weights as the resistance means over machines with integral weight stacks. This is because free weights are often already available within the facility, and machines without weight stacks are generally less expensive to ship and install.
In U.S. Pat. No. 5,044,632, an exercise machine is disclosed in which levers are rotatably mounted to a frame above the seated user. Handles are mounted to the levers. Resistance to handle movement is provided by weight plates mounted to the levers. The hinges for the levers are disposed at an angle of 5.degree. with respect to a central vertical midplane, such that the user must move his hands in defined arcs in particular diverging planes as he presses upward longitudinally on the handles. This apparatus forces the user's hands to follow a preset rate of convergence and divergence during the exercise stroke, regardless of the user's anatomy. This apparatus does not permit the user to select his own path of motion for the press exercise. Rather, the motion is dictated by the angle of the hinges.
A shoulder exercise apparatus is disclosed in U.S. Pat. No. 4,603,856. In this device, a bench is provided for the user to exercise in a prone or supine position. A shaft extends from a ball and socket joint mounted to the side of the bench, and a handle is slidably mounted to the shaft. Frictional resistance is provided both at the ball and socket joint and at the sliding connection between the handle and the shaft. The user exercises by moving the handle against one or both of these resistances. While providing multiple paths of motion through the range of the ball and socket joint, this machine provides for exercising only one arm at a time, cannot coordinate the motion of two arms, and has the disadvantages associated with frictional resistance such as changing resistance due to heat buildup, and wear. Further, this machine only provides concentric action (i.e., where the muscles contract against a load). No eccentric action (i.e., where muscles extend under a load) is possible with this machine.